This invention relates to clock signal generation. In some applications, e.g. in telecommunications testing equipment, clock signal generators are required to produce signals at a set of specific clock frequencies with high accuracy, typically 5 parts per million (ppm). Clock signal generators are known which are based on voltage controlled crystal oscillators. A set of clock frequencies can be produced by providing the clock signal generator with a set of crystal oscillators one for each required frequency. It is also known to provide a small offset from each set frequency by connecting a variable capacitance device in the crystal circuit of each oscillator. By varying the capacitance of the device the frequency of each oscillator can be varied by approximately 100 ppm from its nominal value.
There is a need for a clock signal generator which can produce a wide range of accurately defined frequencies without increasing the number of crystal oscillators. The present invention is concerned with a clock signal generator which aims to meet this need.